The device, the size of a shoe box, is tough to spot unless you make a habit of looking skyward to inspect utility poles while driving.

But in 14 locations around the Washington area, the devices are there, sensing which radio stations drivers are listening to by picking up faint electronic signals emitted from car antennas as they drive by.

The technology, owned by MobilTrak Inc. of Phoenix, was introduced in this region in May. By year-end, MobilTrak hopes to mount nine more units to monitor the listening habits of more than 1 million drivers and present the results to advertisers eager to better reach the audience in the country's eighth-largest radio market.

The monitoring aims to help retailers choose where to advertise by giving them a snapshot of which stations consumers tune into as they drive by their businesses. The most enthusiastic MobilTrak adopters: auto dealers, who generally believe that 80 percent of their business is with people who live or work within 10 miles of a given dealership.

"It's all about precision marketing," said C. David Boice, 39, MobilTrak's managing partner. "It's about giving marketers real-time data about what's happening in certain areas at certain times so they don't waste their advertising dollars."

The approach is the most recent example of the powerful ways marketers are using technology to track customer behavior in natural settings. The strategy in slightly different form is already well entrenched in supermarkets, which track customers' purchases with loyalty cards. Much to the alarm of privacy advocates, technology is helping marketers identify their ideal customers, such as the frequent buyer of a certain brand of detergent, and fine-tune their selling.

In this case, privacy advocates are not too worried because MobilTrak does not collect identifiable data about a car or the person driving it. It cannot see or eavesdrop on the driver. MobilTrak compares its technology to a rubber hose laid across a road to count traffic.

Its solar-powered units randomly pick up signals across six lanes of traffic from cars up to 140 feet away in the same way a police officer measures car speeds by pointing a radar detector at traffic and repeatedly resetting it.

Still, one privacy expert called the technology "creepy" while another raised concerns about the potential of combining it with other technologies to create more intrusive marketing techniques.

"It would be a quick leap to connect that data with other data," said Barry Steinhardt, director of the technology and liberty program at the American Civil Liberties Union. "Technology is moving at the speed of light. We've reached a point where there are few technological bars to doing anything."

Jim Giddings, general manager of Lustine Toyota Scion Dodge in Woodbridge, sees that as progress.

Every month his dealership spends $90,000 on radio advertising. For years, he spent $25,000 on one talk-personality station and another $25,000 on a contemporary music station. Both were recommended by his advertising agency, which consulted ratings from Arbitron Inc.'s research team in Columbia, Md.

Arbitron tracks listening habits by asking small, random samples of people in the nation's major broadcast markets to keep daily diaries. It is the arbiter of how much stations can charge for commercials based on the estimated number of listeners and the demographics those stations attract.

But when Giddings signed on with MobilTrak two months ago, he found that Arbitron's top two stations didn't even rank in the top 10 for in-car radio listeners driving past his dealership. So he shifted his budget, allotting the most money to a news show and a contemporary music station identified as popular by MobilTrak.

"It was a real eye-opener," he said. But is MobilTrak helping him attract more customers? "I don't know yet," he said. "I'll have a better idea 90 days from now maybe."

Thom Mocarsky, a spokesman for Arbitron, described MobilTrak as "complementary" to Arbitron's wide variety of services.

"MobilTrak tells you what your selection of stations should be but it doesn't tell you how many people you are reaching and what you should pay for it," Mocarsky said. "The station with the biggest audience is not necessarily the best buy for a particular advertiser."

A drawback to MobilTrak, Mocarsky said, is that it captures only radio listening in cars, which accounts for 33 percent of all radio listening. Another third of radio listening takes place at work and the rest is done at home. Also, MobilTrak captures only FM stations. But MobilTrak counters that research shows there's no indication that preferences are different at home than in the car. The company also said it plans to introduce technology that picks up AM and satellite station signals next spring.

MobilTrak was founded in Alabama in 1998 by Jim Christian, who once owned the software firm TapScan, which interpreted ratings from radio and TV stations.

Christian, a former radio deejay, sold TapScan to Arbitron in 1998 and sank $10 million to $15 million of his own money into developing MobilTrak's technology, said Boice, the company's managing partner.

The company's early years were devoted to refining the sensors and scaling back their size. (The original was refrigerator-sized and needed electricity.) Along the way, Christian picked up customers.

But MobilTrak ramped up its marketing efforts when Boice and his two partners bought it in April 2004, shortly after selling a Virginia software company they owned.

The three -- Boice, his father Craig Boice, and Kevin Gallagher -- now own 80 percent of the company while Christian owns the rest, Boice said. Christian was not available for comment. Boice and his partners work in Herndon and 15 people work in Phoenix, where the sensors are built. Three MobilTrak salespeople are based elsewhere in the country.

MobilTrak also operates in Seattle, Los Angeles, New Jersey and Charlotte. Within 36 months, Boice hopes to have a presence in 100 markets. One California company, Smart Sign Media, uses MobilTrak's sensors and changes the advertising on digital billboards, depending on which radio station people are listening to as they approach.

The price of the MobilTrak service ranges from $500 to $6,000 a month, depending on the client and the number of locations they want to monitor.

Home Depot just tested the technology in Phoenix, Boice said. Simon Property Group is about to install it in the parking lot of a California mall, he said. And two of the nation's largest radio station owners -- Clear Channel Communications Inc. and Infinity Broadcasting Corp. -- are trying the service in the Washington area.

Radio stations use the data MobilTrak collects to lure advertisers.

For example, if MobilTrak shows that Clear Channel stations have a loyal following in the Tysons Corner area, "that's a chance for me to show all the retailers there that we're a good investment," said Bennett Zier, Clear Channel's vice president for the Washington-Baltimore area.

Michael Hughes, a senior vice president at Infinity, appreciates that MobilTrak can produce ratings information quickly. For example, the firm provided him with data on listeners of a Washington Redskins game on WJFK (106.7) the day after it was broadcast.

Hughes said he knows the data are limited because the information comes from only a few locations around the region. "But media these days is about immediacy," he said. "And to have immediate measurement is very attractive."

MobilTrak has radio sensors in 14 Washington area locations. Car dealers, many of whom have embraced the MobilTrak radio device, above, believe that 80 percent of their business is with customers who live or work within 10 miles of a car lot.